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Not sure what the issue is with the distortion, sounds like it needs a qualified  tech to put it on a bench. As far as loud, what amps are you comparing it to?

Hello everyone, first post.

I have a Red Knob Concert Lead that seems to work fine outside of two issues.

1- The Distortion knob turned up lowers the volume without adding any distortion
     (Distortion switch engaged)

2- The overall volume seems lower than most have suggested, even clean.
     (Can comfortably run the volume on 10 with and without boost engaged)
  It's loud, but not anything close to any of my other amps.

Everything else seems fine.

Selling a very nice Concert Bass from the "red knobs" era - amp has been serviced and works without issues. $375 shipping can be negotiated but amp weighs almost 40 lbs without packaging.

Selling a very nice Concert Lead - date code 12/16/1973 so this is a black/silver front panel amp. The amp has been serviced so every thing works as designed and I will  provide a short RCA jack so you can enable the distortion feature. $350 and shipping can be negotiated but it will be $$ these days (amp weighs close to 30 lbs without packaging).

Thank you so much, just what I'm looking for!

You should be able to find what you need here:

https://forum.sunnstillshines.online/index.php?topic=6794.msg29078#msg29078

I am in need of a Sunn Stage Bass schematic. Picked up a 115 combo for $50, and was running a speaker cable from the head section out to the cab input, not knowing that the head was already internally wired to the speaker (amp would not produce sound without the speaker cable in between). Sounded great for about 30 minutes and then the sound died off. Had this problem before with the amp, but was always able to get it to produce sound again. Now it's just plain ol' dead. After taking it to my dad, who works on amps and guitars, there were no obvious signs of damage so we need a schematic of the amp or something similar to do a full troubleshoot. Thanks!

Some stoner/doom players tune their guitars down to C so ordinary guitar speakers might not be up to the task. A pair of 4x12 cabinets might not be enough. It might take four of 'em.

Never mind, I purchased 7 from a fellow on Reverb 

The fellow didn't give them away, they were $15 a piece negotiated. The knobs I received are used but in terrific condition with bright numbers, no wear and no cracks. They shipped right away with tracking info. I bought 7 for my Sentura II project and I see he must have more as he is still running the auction. These don't appear often and when they do they are usually in kind of crappy condition so I thought I would share the URL.

https://reverb.com/item/10017254-sunn-o-model-t-knobs-c-1973-set-screw-original-vintage-usa

ps. I haven't cleaned the ones I received yet.

Musings, which may or may not be applicable.

Guitar speakers are almost always louder than bass speakers, except in the lowest register. It's basic acoustics: it takes more energy to move air at low frequencies than at higher frequencies, so guitar speakers, and many guitar amps, simply don't bother. In addition, human hearing is most sensitive around 1-2KHz, so a lot of guitar speakers are optimized to have their greatest output there. That makes them sound louder, even if they're not.

I'm not familiar with the stoner/doom genre, so I don't know what kind of sound you're going for. Maybe bass speakers are more likely to get you that sound, but I doubt it.

More speakers usually gets you more volume faster than more power.  The silver face Concert amps were stable only down to 4 ohms. The black face models claimed stability down to 2 ohms, but I have read knowledgeable people saying that the circuit topology doesn't support that, so I wouldn't trust it at 2 ohms. So, if I wanted more volume, I'd try to use two 8 ohm cabinets or as many as four 16 ohm cabinets, depending on what I had available and could transport.